diff --git a/image_analysis/beam_stop/ShadowFinder.cpp b/image_analysis/beam_stop/ShadowFinder.cpp
index 59a65d9b..b289ab46 100644
--- a/image_analysis/beam_stop/ShadowFinder.cpp
+++ b/image_analysis/beam_stop/ShadowFinder.cpp
@@ -328,10 +328,14 @@ std::vector<uint32_t> ShadowFinder::GetMask() const {
     // Counting statistics cannot decide this: on a bright dataset the shadow is still well counted.
     // The comparison is only ever used to answer "is this whole ring inside the stop", never to
     // judge an individual pixel, so taking the largest background over an outward window is safe
-    // here in a way it would not be per pixel.
+    // here in a way it would not be per pixel. It is taken only over the rings this same walk is
+    // willing to judge, though: at the corner of the detector a ring holds a handful of pixels and
+    // its median is one pixel's mean, so one recorded reflection out there would otherwise become
+    // the background every ring inside it is compared against.
     std::vector<float> outward_max(max_radius + 2, 0.0f);
     for (int rad = max_radius; rad >= 0; rad--)
-        outward_max[rad] = std::max(baseline[rad], outward_max[rad + 1]);
+        outward_max[rad] = std::max(ring_pixels[rad] >= MIN_RING_PIXELS ? baseline[rad] : 0.0f,
+                                    outward_max[rad + 1]);
 
     int blocked_out_to = -1;
     for (int rad = 0; rad <= max_radius; rad++) {